1. Field of the Invention
The present invention relates to a process for causing continuous severe plastic deformation of metals, and in particular for creating nanostructured metals.
2. Description of Related Art
Bulk nanostructured metals (nanometals) attract substantial attention due to their unique mechanical and physical properties. For example, at low temperatures, an ultra fine grain size (<1 μm) doubles the strength and toughness of the material and, at high temperatures, it leads to superplastic behaviour at the strain rate which is one order higher than for traditional superplastic materials. The preferred method of producing bulk nanometals, which avoids health issues associated with nanopowders, is severe plastic deformation (SPD). In this, a very large plastic deformation (true strain 3-10 depending on the material) subdivides the coarse grain structure of all types of metals into sub-micron and nano grain structure. SPD processes are different from traditional metal forming processes by their ability to retain the shape of the workpiece.
There are two groups of SPD processes—batch and continuous processes. Batch processes deal with relatively short billets with a limited length to width ratio (about 6). They are usually used for laboratory purposes to produce samples for further tests. The most popular batch process is Equal Channel Angular Pressing (ECAP) also known as Equal Channel Angular Extrusion (ECAE). Examples of this are described in U.S. Pat. No. 5,400,633, U.S. Pat. No. 5,513,512, U.S. Pat. No. 5,600,989, U.S. Pat. No. 5,850,755, and U.S. Pat. No. 5,904,062. In this process, a rectangular or cylindrical bar is pushed from one section of a constant profile channel to another section orientated at an angle ≧90° to the first one, as shown in FIG. 1. Plastic deformation of the material is caused by simple shear in a thin layer at the crossing plane of the channel sections. However, a problem with this technique is that the ideal mode of deformation shown in FIG. 1 cannot be achieved in practice because of end effects and non-uniform strain distribution across the channel. Another problem is that the length of the leading channel limits the length of the billet. It must not be too long to avoid an excessive force caused by friction and the associated tool design problems.
There will be cases when a batch process is technically justified and economically viable. However, for high volume production of variety of nanostructured metals a continuous process would be much more valuable to industry. Such a process could be a real breakthrough and allow production and implementation of nanostructured metals on a large scale.
Various continuous SPD processes have been proposed. Some of these are derived from the so-called Conform process. This is described by Y. Saito, H. Utsunomiya, H. Suzuki: in M. Geiger (Ed), Advanced Technology of Plasticity, Springer, 1999, Vol. III, pp. 2459-2464; J. C. Lee, H. K. Seok, J. H. Han, Y. H. Chung, Mater. Res. Bull. 36 (2001), 997-1004 and G. J. Raab, R. Z. Valiev, T. C. Lowe and Y. T. Zhu, Mater. Sci. Eng. A328 (2004), 30-34. The original Conform process was not intended for nanostructuring. This is a continuous lateral extrusion process with the material led to the extrusion chamber by a grooved wheel and constrained by an abutment, as shown in FIG. 2. Due to intensive deformation and friction in the leading channel, the material reaches the chamber hot enough to be easily extruded. However, a significant problem with SPD processes based on Conform is that the force required to extrude the material is relatively high. Since feeding of the workpiece is based on friction, this leads to heating up of the material. Whilst this is a virtue in the original Conform process, because high temperature leads to grain growth, it is a potential problem in an SPD process.
Equal Channel Angular Drawing (ECAD) is another proposed continuous SPD process. This is described by A. B. Suriadi and P. F. Thomson in Proc. of Australasia-Pacific Forum on Intelligent Processing & Manufacturing of Materials, IPMM, 1997, pp. 920-926. In this, the workpiece is pulled through a die, as shown in FIG. 3. The pulling force in ECAD is limited by fracture of the drawn workpiece. This can only be avoided by the high workpiece/die clearance. A problem with this is that it results in a change of the character of the process from the most effective mode of simple shear to bending combined with tension.
Another proposed technique is accumulated roll bonding (ARB). This is described by Y. Saito, N. Tsuji, H. Utsunomiya, T. Sakai, R. G. Hong, in Scripta Mater, 39 (1998) No. 9, 1221-1227. In this a rolled sheet is cut, cleaned, stacked and hot rolled again, as shown in FIG. 4. This sequence is repeated several times until a desired strain is achieved. Because many operations are involved, ARB is not a true continuous process. It is limited by the manageable sheet size. The success of the process depends critically on the quality of the bond, which could be difficult to achieve. The microstructure of metals subjected to ARB is not uniform (a layered structure) and grains are elongated due to rolling.
U.S. Pat. No. 6,197,129 B1 describes another proposal. This is referred to as repetitive corrugation and straightening (RCS). RCS involves bending of a straight plate/bar between corrugated rolls and then restoring the straight shape of the plate/bar with smooth rolls, as shown in FIG. 5. A problem with this process is that it does not use simple shear, and bending leads to non-uniform strain distribution across and along the workpiece.